1. Field of the Invention
The present invention relates to a self-compensating laser resonator for compensating for the inclination of a reflecting surface, used in, for example, a solid-state laser apparatus which is provided in a flying object such as a satellite or aircraft.
2. Description of the Related Art
FIG. 26 is an explanatory drawing showing a conventional laser resonator shown in, for example, the Springer Series in Optical Sciences Vol. 1, “Solid-State Laser Engineering” Fourth Edition 1995, page 197. In FIG. 26 a pair of reflecting mirrors 1, 2 for confining a laser beam are disposed opposite each other, a laser medium 3 is disposed between reflecting mirrors 1 and 2, an excitation light source 4 excites the laser medium 3, 5 denotes a laser resonator optical axis, a laser beam optical path 6 extends from the reflecting mirror 2 to the reflecting mirror 1, a laser beam optical path 7 extends from the reflecting mirror 1 to the reflecting mirror 2.
Next, the operation will be explained. In a laser resonator structured as above, a laser beam travels back and forth along the optical paths 6, 7, and only the laser beam which travels back and forth along the same optical paths 6, 7 and maintains the same phase state is selectively confined and amplified so as to form an oscillation mode. When the reflecting mirrors 1 and 2 are disposed parallel to each other, the laser beam is repeatedly reflected between the reflecting mirrors 1 and 2 to travel back and forth along the optical paths 6, 7 which are parallel to the optical axis 5. At this time, the laser beam repeatedly passes through the laser medium 3, which is excited by the excitation light source 4, so as to be gradually amplified.
On the other hand, when the reflecting mirror 1 is inclined so that the reflecting mirrors 1 and 2 are not parallel, as shown in FIG. 27, the laser beam which travels along the optical path 6 parallel to the optical axis 5 is reflected along an optical path 8 at a angle relative to the optical axis 5. Accordingly, the laser beam does not travel back along the same optical path and an oscillation mode can not be formed.
As a means for overcoming this problem, a self-compensating laser resonator, such as that shown in FIG. 28, is also shown on page 227 of the above “Solid-State Laser Engineering.” In FIG. 28, a roof prism 11 has a ridge 11a which is parallel to the Z axis, a roof prism 12 has a ridge 12a which is parallel to the X axis, a corner cube prism 13 is disposed facing the roof prisms 11 and 12, a laser medium 3 is disposed between the corner cube prism 13 and roof prism 11.
In a laser resonator constructed as above, a laser beam travels along an optical axis 14 and is reflected by the roof prism 12. The laser beam reflected by the roof prism 12 is bent back by the corner cube prism 13 and passes through the laser medium 3 excited by the excitation light source 4 so as to be amplified. The laser beam now amplified by the laser medium 3 is reflected by the roof prism 11 to be amplified again by the laser medium 3. Then, the laser returns to its original position so as to be confined inside the laser resonator.
Next, FIG. 29 is an explanatory drawing showing the reflected state of the laser beam incident upon the roof prism 12 of FIG. 28. In FIG. 29, reflecting surfaces 12b, 12c having the ridge 12a therebetween are set at a right angle to each other and form 45 degree angles with the optical axis 14. A laser beam, which travels along an optical path 15 parallel to the optical axis 14 to be incident upon the roof prism 12, is bent 90 degrees by the reflecting surface 12b and bent 90 degrees by the reflecting surface 12c so that the direction thereof is changed a total of 180 degrees. An optical path 16 of the laser beam reflected in this manner is parallel to the optical axis 14. That is, the roof prism 12 reflects the incident laser beam as a laser beam which is parallel to, and travels in the opposite direction of, the incident laser beam.
Also, as shown in FIG. 30, when the roof prism 12 is tilted an angle α with the ridge 12a as the center axis, the angle provided from the reflecting surface 12b is changed by 90+2α, where as the angle provided from the reflecting surface 12c is changed by 90−2α. Thus, the total change in angle between the laser beam traveling along the optical path 15 and the optical path 16 of the laser beam reflected by the roof prism 12 is 180 degrees. Accordingly, even when there is inclination with the ridge 12a as the center axis, the roof prism 12 reflects the incident laser beam as a laser beam which is parallel to and travels in the opposite direction of the incident laser beam.
Furthermore, the roof prism 12 reflects the incident laser beam as above, as a laser beam which is parallel to and travels in the opposite direction of the incident laser beam, in the case where the incident laser beam is inclined with respect to the optical axis 14 as well. Moreover, for the sake of explanation, the optical path 15 and optical path 16 are shown shifted from the optical axis 14 in the FIGS. 29 and 30. However, in actuality, the center of the beam corresponds to the optical axis 14 in both the optical path 15 and the optical path 16, and the laser beam is emitted and reflected in a range that includes the ridge 12a. 
Still further, as shown in FIG. 28, a self-compensating laser resonator is constructed by disposing the ridge 11a of the roof prism 11 and the ridge 12a of the roof prism 12 in orthogonal directions to each other so that the inclination of the roof prism 11 and the roof prism 12 compensate each other.
As configurations of the laser resonator, a traveling wave resonator and standing wave resonator will be considered here. With a traveling wave resonator, a laser beam emanating from a laser medium returns to the laser medium without taking the same optical path, during one circuit inside the resonator, so as to be confined inside the laser resonator as a traveling wave. At this time, during one circuit in the resonator, the laser beam passes through the laser medium one time and is amplified. On the other hand, with a standing wave resonator, a laser beam emanating from a laser medium returns to the laser medium by traveling back along the same optical path so as to be confined inside the laser resonator as a standing wave. At this time, during one circuit in the resonator, the laser beam passes through the laser medium twice and is amplified. That is to say, when a laser beam circuits the resonators once, with the traveling wave resonator it is only amplified one time, whereas, with the standing wave resonator it is amplified twice. The laser resonator shown in FIGS. 26, 28 is a standing wave resonator.
In a conventional self-compensating laser resonator configured as described above, since the laser beam is emitted in a range that includes the ridges 11a and 12a of the roof prisms 11 and 12, loss occurs due to diffraction and the efficiency of the laser beam is degraded. Also, since, on a microscopic level, the kind of reflection shown in FIG. 29 does not occur at the ridges 11a and 12a and the laser beam is split by the ridges 11a and 12a, the laser beam is easily split into two (2) or four (4) and the quality thereof is degraded. Furthermore, when attempting to realize a long optical path with a small laser resonator, an optical element for bending back the laser, such as the corner cube prism 13, is needed in addition to the roof prisms 11 and 12, and thus, the structure of the laser resonator becomes complicated. Moreover, the roof prisms 11 and 12 apply different phase shifts to the laser beam field component parallel to the ridges 11a and 12a (S polarization) and the laser beam field component orthogonal thereto (P polarization). Therefore, except for the instances where the linearly polarized laser beam is reflected parallel or orthogonal to the ridges 11a and 12a, the polarization of the laser beam reflected by the roof prisms 11,12 is not maintained such that the phase state of the laser beam having made a circuit in the resonator is not harmonized and loss occurs. Consequently, the utilization efficiency and quality of the laser beam are degraded.